Basal level of ATF4 promotes T cell readiness for activation-induced proliferation.

Stress-response elements are required during late-phase T cell activation and differentiation. To investigate whether they are indispensable during the first 12-24 h post-stimulation when mitochondrial activation and metabolic reprogramming are critical, we activated Atf4-sufficient and Atf4-deficient T cells and tracked their earliest activation dynamics. We demonstrate that T cell activation-induced mTOR and GCN2 phosphorylation leads to the upregulation of ATF4 protein as early as 12 h after stimulation. This early induction of ATF4 has transcriptional activities that regulate stress response, signaling, and metabolism. Loss of Atf4 in T cells alters transcriptome dynamics, impairs amino acid transport and biosynthesis, and disrupts adaptive responses to ER stress and oxidative stress, resulting in defective effector cell differentiation in vitro or in vivo. Our findings suggest that a basal level of ATF4 during the early phase of T cell activation enhances the preparedness of cells to cope with integrated stresses during the activation course.